The vulnerable situation of coastal lowlands with respect to flooding is aggravated by land subsidence due to groundwater withdrawal. Because of the growing demands of the ever increasing population, industry and tourism, excessive and uncontrolled pumping is the order of the day in most of the great coastal cities. Therefore, well-balanced groundwater management has become imperative for the survival of these sinking cities. Groundwater management implies geotechnical and hydrogeological research. The geological data, however, still form the basic framework, but geological data can only be useful if the three-dimensional spatial extension of relevant units is delineated. The elaboration of the geometry based on the knowledge of processes responsible for the deposition of the sediments and the formation of their facies is analysed. The analysis is illustrated with the case study of land subsidence in Shanghai (China).